Two ASN's needed in this configuration?

Posted on 2005-05-17
Last Modified: 2010-04-10
Here's my proposed config:

                                             --------------2851 2 T1's bonded to ISP A =======================> Internet
PIX 520 ------- Catalyst 3500XL|                                        | (interconnected for BGP exchanges)                                        
                                             --------------2851 2 T1's bonded to ISP B =======================> Internet

Currently we have a single ASN for one multihomed router.  For redundancy and load balancing, we're looking to utilize GLBP with the topology above.  So, if I plan to do this, would I need a second ASN?


Question by:cory_spence
    LVL 27

    Expert Comment

    Dude, we just (an hour ago) tossed about 5 ASN's in the dumpster.

    Where is the ASN in your diagram?  

    I like your diagram the way it is.

    ASNs BAD.
    LVL 32

    Expert Comment

    No, you should continue to use one AS. You want to originate your prefix advertisement from one source via BGP.
    I assume you have a /24 to advertise otherwise your route is being sumarized by your upstream provider. There are some complex scenarios in which multiple ASs are used but this is not one of them. GLBP is for your out going traffic only, your return path is not guareteed to come back the same way.


    Author Comment

    Just got off the phone with Cisco - they responded pretty quickly this time!

    Anyway, they said pretty much the same thing as you, harbor235.  Both routers advertise the same networks (two class c's that we own) by using the same ASN.  Then we'll run iBGP inbetween the two using the 'next hop self' command.  Harbor235, I'm going to award the points to you, but would you mind explaining to me what the next hop self command and how it will be helpful?  

    Also, are there any 'gotcha's' to this config/topology?


    LVL 27

    Expert Comment

    OIC!  I'm sorry, I didn't read "Autonomous System Number", I read "Access Stack Nodes" which are OLD Bay routers.  Sorry to be so DENSE! :)
    LVL 32

    Accepted Solution

    BGP routes have numerous attributes associated with them to aide in the route selection process. The next-hop attibute is one of those attributes. IBGP speaking routers maintain the EBGP next hop attribute received from an advertising EBGP peer. So for example:


    When Router1_as2 receives BGP routes from Router1_as1 each route has a next-attribute associated with it, and it will be the IP address of the interface common between them. IBGP will carry that same next hop address to other IBGP speaking routers in AS2, in this case Router2_as2_IBGP, and Router3_as2_IBGP.  That is why it is important to have an internal route to the next hop. The BGP route would be unreachable otherwise. To make sure you can reach the eBGP next hop, include the network that the next hop belongs to in a IGP like OSPF or use the next-hop-self neighbor command to force the router to advertise itself. This is the way BGP works
    and failure to configure BGP properly will result in BGP routes not being installed into the routing table. I hope yhis is clear, if not check out the links below.

    Here are some links:


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